If I wanted to buy a car over the border in Canada (including a Chinese-made car) or Mexico and drive it home what would be the car importing rules?

If the car is less than 25 years old, it needs to be certified compliant with US safety and emissions regs, to be registered in the US.

Then there is the pretty well scrambled tariffs.

In a flight of fancy, I considered importing a vintage VW from Brazil. There are companies that do turnkey service for that, and old cars are not subject to US regs. I started following "Kombi Warriors" which is one of the exporting companies, on FB. One day, they posted a pic of a Toyota HiLux pickup they were loading, bound for Houston. With the court overturning some of the US' country specific tariffs, I wondered what the current total hit is, so I asked them. Current total US tariff load on a car from Brazil is 52%.

I'm a libertarian in many ways so I don't think government should get involved in a personal buying decision but I know that's naive.

Well, when you are talking about a car, you are using public roads and public air, that are shared with other people, so it becomes the government's business.

Steve